1. Introduction
It is difficult to attain bandwidth greater than 10% of the operating frequency from a single radiating element. Tapered slot antennas have been reported (see the Lee and Livingston article cited below) to achieve broadband operation; however, for use in an array, the size of the radiating elements in the array would be greater than ½ λ at the highest frequency of operation, resulting in grating lobes, else the size of the radiating element is too small at the lowest frequency, resulting in a very difficult impedance match. The problems of impedance matching and array spacing are further exacerbated when these elements are arrayed for dual polarization.
The present disclosure relates to an element card for an ultra wideband array antenna. Ultra wideband operation is achieved by using multiple radiating elements, each optimized for a particular frequency band. These radiators are then integrated onto a single element card. In addition, high gain radiators are preferably used, which have thin cross-sections, so that the elements can be placed close together with minimal mutual coupling. Since the element cards are fabricated with individual radiators, cards only need to include those radiators necessary to maintain grating free spacing operation, thus resulting in a thinned array and reduced cost and weight.
J. J. Lee and S. Livingston in “Wideband bunny-ear radiating element,” Antennas and Propagation Society International Symposium, 1993 AP-S Digest, 1993, pp. 1604–1607, describe a wideband flared notch printed circuit radiation element for operation from 0.5–18 GHz. While the element achieves 36:1 bandwidth, its use in an array is severely limited in bandwidth to less than 2:1 because the element size is greater than ½ λ at the highest frequency.
The element card disclosed herein uses high gain dielectric rod antennas at the higher frequencies, and preferably a small TEM horn at the lower frequencies. Radiating elements of the present invention can be placed much closer together than for the flared notch, and each radiator can be impedance matched separately rather than trying to do an ultra wideband impedance match. The multiplexing of signals of the element card disclosed herein can be done in the beamformer using standard multiplexing microwave circuits. The dielectric rod antennas may be cladded so that they are operable in multiple frequency bands.
Adrian E. Popa and William B. Bridges in U.S. Pat. No. 6,266,025 dated Jul. 24, 2001 and entitled “Coaxial Dielectric Rod Antennas with Multi-Frequency Collinear Apertures” describe the use of dielectric rod antennas with core and cladding cross-sections to achieve wide bandwidth from a radiating element. The feed structure disclosed in that patent includes collinear round waveguides, which are 1) limited in bandwidth, and 2) not easily integrated with low-cost printed circuit feed circuits.
The present disclosure improves on this prior art by teaching how to make low-cost printed circuit cards that can be integrated with one or more uncladded or cladded dielectric rod antennas. Furthermore, the present disclosure demonstrates how other types of transmitting and/or receiving structures, such as TEM horn antennas, can be integrated therewith to form an ultra wideband element card radiator and/or receiver. In addition, the present invention shows how to use these cards in beam steering arrays.
Albert D. Krall and Albert M. Syeles in U.S. Pat. No. 4,274,097 dated Jun. 16, 1981 and entitled “Embedded Dielectric Rod Antenna” present a dielectric rod antenna that is surrounded by a lower dielectric constant material. It is used to make the dielectric rod antenna compact. It is not the same arrangement as U.S. Pat. No. 6,266,025, above. For example, the surrounding cladding material is not tapered. It suffers from difficulty in feeding and is not compatible with printed circuit technology.
None of these prior art references address how to utilize their antenna elements in an ultra wideband, low cost array.
2. Dielectric Rod Antennas
Dielectric rod transmission lines and antennas have been studied for more than 60 years. Some advantages of using a dielectric rod antenna over metallic elements or other dielectric based antennas, particularly for microwave and millimeter wave frequencies include:                1) Large effective aperture—In volumetric, traveling wave type antennas such as the long Yagi, the helix and the dielectric rod, antenna gain is a function of the length of the antenna in the direction of wave propagation along the antenna rather than the transverse dimensions of the antenna. This means the effective area ArM is much larger than its physical transverse cross section.        2) Low-cost manufacturing—Dielectric rod antennas can be fabricated through molding techniques, and integrated onto printed circuit boards. A transition from microstrip into the dielectric rod antenna facilitates matching the dielectric rod antenna to active components such as amplifiers, lasers, or mixers.        3) Ease of integration with other antenna components—Since the dielectric rod antenna can be integrated onto a printed circuit board, it can also be mechanically integrated with other printed circuit antennas. The small physical aperture for a dielectric rod antenna with high gain (7–20 dB) helps to mitigate mutual coupling effects with these other antennas.        
Additionally, at millimeter wave frequencies, the dielectric rod antennas will have lower loss compared to metal based printed circuit antennas such as notches and dipoles (i.e. Yagi or vee type antennas).
The basic dielectric rod antenna, shown in FIG. 1, provides a unique transmission line antenna that has a number of features and benefits that can be exploited for optimizing large diameter (narrow beamwidth), wide bandwidth (multi-octave), wide field-of-view (FOV), phased array antennas. The directivity of the dielectric rod antenna is a function of the length of the dielectric rod. For maximum directivity, the base diameter D should be:   D  =            λ      0                      π        ⁡                  (                                    ɛ              r                        -            1                    )                    
Past designs for dielectric rod antennas have focused on maximum on axis gain in a narrow frequency band, and in fact, “information on the bandwidth of tapered-rod antennas is scarce” as disclosed in F. Schwering and A. A. Oliner in “Millimeter-Wave Antennas” Antenna Handbook, Volume III, Y. T. Lo and S. W. Lee, eds., Chapman and Hall, New York, 1993, pp. 17–44. Since there is neither low frequency cutoff for the HE11 mode on the dielectric waveguide, nor any high frequency limit, the bandwidth of an antenna using dielectric waveguide is, in principle, unlimited. In practice, however, the bandwidth is limited for a given desired gain on the low end by excessive wave leakage. On the high frequency end, it is usually limited by the appearance of higher order modes of transmission in addition to the fundamental HE11 mode. Of course, the bandwidth of the dielectric rod antenna can also be limited by the feed structure unless it is specifically designed to have broad bandwidth as well. For example, the “Polyrod” antennas of World War II were fed by resonant microwave cavities, and exhibited quite narrow bandwidths. For waveguide fed antennas, the usable bandwidth approaches approximately 2:1, and a 3:1 bandwidth antenna has been recently reported in Chi-Chih Chen in “Novel Wide Bandwidth Dielectric Rod Antenna for Detecting Antipersonnel Mines,” IEEE Geoscience and Remote Sensing Symposium 2000 Proceedings, IGARSS 2000, Vol. 5, pp. 2356–2358. Dielectric rod surface wave antennas can be designed for omnidirectional applications or for end-fire applications with gains up to 20 db. See J. D. Krause, Antennas, McGrall-Hill, 2nd Ed. 1988.
To extend the bandwidth of a dielectric rod antenna, a new collinear, coaxial dielectric rod antenna was invented. See U.S. Pat. No. 6,266,025. The coaxial dielectric rod antenna, shown in FIG. 2, includes a lower frequency range dielectric rod antenna with a tapered radiating aperture with an embedded higher frequency band coaxial dielectric transmission line terminating in a second dielectric rod antenna radiating aperture. Each radiating rod can be designed for optimized gain patterns and the high band antenna is designed with a low frequency cutoff near the highest operating frequency selected for the low frequency band antenna.
The structure, shown schematically in FIG. 2, consists of a dielectric rod 201 inside a tapered dielectric cylinder 202 of somewhat lower dielectric constant. The tapered end 203 of the central rod 201 is the radiating structure for higher frequencies (i.e. for a higher frequency band) while the tapered cylinder 204 plus the central rod 201 together is the radiating structure for lower frequencies (i.e. for a lower frequency band). The antenna structure can have additional dielectric structures to thereby increase the number of different radio frequency bands served by the dielectric rod antenna 501. Generally speaking, the TEM horn antenna 502 serves a lower frequency band than the band(s) served by the dielectric antenna 501.
The outer cylinder 202 serves as a cladding around the inner core 201, which forms a non-radiating transmission line for an upper octave. Even though the embedded inner core 201 has no low frequency cut-off, the cladding layers help to contain the electric field density at low frequencies for guidance to the radiating taper 202. At higher frequencies, the electric field is constrained to be more in the higher dielectric constant core 203. The antenna feed may operate as a single mode waveguide up to the next higher order mode cut-off frequency, which should lie between the next higher mode cut-off frequency of a homogenous cylindrical waveguide of the cladding layer diameter and the next higher mode cut-off frequency of a homogenous cylindrical waveguide of the core region. The result is an embedded dielectric rod antenna with a diameter of the outermost cladding layer that has an extended operational frequency than could be obtained with a homogeneous material dielectric rod antenna. Separate metallic feed structures 206, 207 (shown conceptually in FIG. 2 as metal waveguides, which limit the bandwidth to a single octave for each feed) feed each radiator.
3. TEM Horn Antennas
At RF and low microwave frequencies, the width of dielectric rod antennas becomes large and another type of antenna must be integrated into the broadband card to keep the size and weight of the card as little as possible. One antenna that can give relatively large bandwidths is the transverse electromagnetic (TEM) horn antenna. Basically, a TEM horn 502 is just a horn antenna, but with the sides removed. Generally these antennas are fed by parallel plate waveguide and do not need to be integrated onto printed circuit boards 500 with the other dielectric antenna elements 501.
4. Array Thinning
This information is included for a better technical understanding of some of the array aspects of the present invention to be discussed later. A receiving antenna will pick up energy from an incident plane wave and will feed it into a transmission line that terminates in an absorbing load, such as a detector, mixer or low noise amplifier. The amount of energy absorbed in the load will depend on three factors, the orientation of the antenna, the polarization of the wave, and the impedance match in the receiving system. If these factors are set for maximum power absorbed, the absorbed power can be expressed as an effective receiving cross-sectional area ArM of the antenna.
The maximum gain GM of an antenna is the greatest factor by which the power transmitted in a given direction can be increased over that of an isotropic radiator. As a consequence of the reciprocity theorem it can be shown that the ratio ArM/GM is constant for all matched antennas:ArM/GM=λ2/4 π                Where:        ArM is the maximum effective receiving area        GM is the maximum gain        λ is the wavelength        
The implication of this result is that ArM is a function of the gain and the wavelength, and while ArM can be approximated by the physical aperture for many planar antennas, this is not true for many three dimensional volumetric antennas in common use. In volumetric, traveling wave type antennas, such as the long Yagi, helix and dielectric rod, the gain is achieved in the direction of wave propagation on the antenna which can significantly increase the effective receiving cross-sectional area ArM beyond the physical aperture of the elemental antenna in the plane of an array as demonstrated in FIGS. 3a and 3b. This increase in effective aperture and the subsequent ability to reduce of the number of elements in the physical aperture is known as array thinning If the pattern of the elemental antenna can be designed to fill the field-of-view (FOV) of the electronically steered array, elemental antenna gain can be used to increase the effective aperture and to reduce (thin) the number of elements in the physical aperture. This thinning is illustrated in FIG. 4 and tabulated in Table I for several FOVs.
TABLE IField ofElementArray ElementViewGain OverThinning Over(FOV)DirectivityDipoleDipoles λ/2 Spacing 60°8.99.5 dB89% 70°6.98.4 dB85% 80°5.47.3 dB81% 90°4.36.3 dB76%100°3.55.5 dB72%110°3.04.7 dB66%120°2.54.0 dB60%Biconical1.50.0 dB 0%Dipole